I have simply calculated the shear resistance for a bent up (diagonal) bar per the given Vs equation in ACI 318...the problem is, this only applies in the region of the bent up bar.
Historically, old structures that used this bent up bar system didn't really depend on this bar all that much for shear. Either side of the diagonal portion of the bar the Vs goes to zero unless there are additionally spaced stirrups.
Unless i have misunderstood, I assume you refer to main longitudinal bars bent up diagonally to provide shear resistance. I am not so sure about "historically old structures". In the UK and europe it has and continues to be quite a common and effective method of providing shear resistance at the points where shear is usually at maximum, ie supports, and main tension reinforcement can be bent up and utilised for shear. Stirrups would still be required throughout as there is a limit for inclined bars to provide up to half of the shear resistance required. Inclined bars have also been used to avoid clutter or impractical stirrup spacing at these locations.
xxpegasus11 -
I am in the US and was stating the historical case based on the fact that here these types of bent bars are not specified much (if at all) anymore. Much to much labor intensive.
It's interesting to know that in Europe its still utilized. Am I right, though, in stating that only the small length of beam located in the range of the diagonal portion is helped in shear? And that either side of the diagonal portion is not helped by these bars?
If I understand JAE correctly, in my case, I will need to provide bent up bars until the shear capacity of the concrete alone can take the shear. Is this correct thinking?
I would suggest that you use the British BS8110 rule as a minimum guide, if your code doesn't stipulate otherwise.
As posted by xxpegasus11; "Stirrups would still be required throughout as there is a limit for inclined bars to provide up to half of the shear resistance required."
Guess its a matter of economy and whats practical. You wouldnt normally find inclined bars in simple rc beams but still there on deeper beams, heavy shear etc.
The maximum effect would be taken over a greater length than just the diagonal portion though. The bars are assumed to act as tension members of a lattice girder, with concrete providing the compression struts. So, in the simplest case say, of one inclined bar at 45deg, the top "bend" would be 0.41d from support,plus 1.41d being the distance of the imaginary V formed by the bar and the concrete , so in effect the total length for shear resistance provided would be 1.82d
Sorry, if it sounds confusing - hard to describe without a sketch.
Brian,
The excess shear over the concrete shear resistance can be provided with stirrups or a combination of stirrups and inclined bars. Your code I expect has a limit on how much can be taken by inclined bars. In the UK it would be 50% if thats any help.
Thank you all for your help. We have much experience in our office, but there are still items we have no experience with. If is great to have this resource. Keep up the good work.
Like JAE said, bent bars are not used in the US for economical reasons (labor). A great majority of countries still use bent bars. Talking to a former professor from India, it is still used in every country he has worked with. Especially in those countries where labor costs are still low and the structural benefit of the bent bars can be economically justified. It may still be in the ACI due to those rare cases where bent bars are the only solution (for whatever the reason). Once again, if you are in the US, do not use bent bars unless necessary.
